In general, a compressor is a mechanical apparatus for raising a pressure, by receiving power from a power generation apparatus such as an electric motor or turbine, and compressing the air, refrigerants or other various operation gases. The compressor has been widely used for an electric home appliance such as a refrigerator and an air conditioner, or in the whole industry.
The compressors are roughly classified into a reciprocating compressor in which a compression space for sucking or discharging an operation gas is formed between a piston and a cylinder, and the piston is linearly reciprocated inside the cylinder, for compressing refrigerants, a rotary compressor in which a compression space for sucking or discharging an operation gas is formed between an eccentrically-rotated roller and a cylinder, and the roller is eccentrically rotated along the inner wall of the cylinder, for compressing refrigerants, and a scroll compressor in which a compression space for sucking or discharging an operation gas is formed between an orbiting scroll and a fixed scroll, and the orbiting scroll is rotated along the fixed scroll, for compressing refrigerants.
Normally, the linear compressor sucks, compresses and discharges the refrigerants by using a linear driving force of a motor, and is divided into a compression unit including a cylinder and a piston for compressing the refrigerant gas, and a driving unit including a linear motor for supplying the driving force to the compression unit.
In detail, in the linear compressor, the cylinder is fixedly installed in a hermetic container, and the piston is linearly reciprocated in the cylinder. As the piston is linearly reciprocated inside the cylinder, the refrigerants are supplied into a compression space in the cylinder, compressed and discharged. A suction valve assembly and a discharge valve assembly are installed in the compression space, for controlling suction and discharge of the refrigerants according to an inner pressure of the compression space.
The linear motor for generating the linear driving force is connected to the piston. In the linear motor, an inner stator and an outer stator formed by laminating a plurality of laminations in the circumferential direction are installed around the cylinder with a predetermined gap, a coil (or coil winding body) is wound around the inner stator or the inner portion of the outer stator, and a permanent magnet is installed in the gap between the inner stator and the outer stator and connected to the piston.
The permanent magnet is movable in the motion direction of the piston. The permanent magnet is linearly reciprocated in the motion direction of the piston by an electromagnetic force generated when a current flows through the coil. The linear motor is operated at a constant operating frequency fc, and the piston is linearly reciprocated at a predetermined stroke S.
FIG. 1 is a circuit view illustrating a conventional control apparatus for a linear compressor. Referring to FIG. 1, the control apparatus includes a coil winding body L wound in the circumferential direction of the linear compressor, for receiving power, a branch means 100 for applying power to the part or whole of the coil winding body L, and a control unit 200 for controlling the branch means 100 to control a cooling force according to a load.
In detail, a power supply source is connected to one end of the coil winding body L, and a connection terminal 100a of the branch means 100 is formed at the other end of the coil winding body L. A connection terminal 100b is connected to a middle point M (or a branch line of the middle point L) of the coil winding body L. The branch means 100 includes a switching element 100c for applying power to the connection terminal 100a or 100b by the control of the control unit 200.
The control unit 200 performs a power mode of applying power to the part of the coil winding body L in order to output a high cooling force in an overload of a freezing cycle, and performs a saving mode of applying power to the whole coil winding body L in order to output a low cooling force or a middle cooling force in a low load or mid load of the freezing cycle. For the power mode, the control unit 200 connects the switching element 100c of the branch means 100 to the connection terminal 100b. For the saving mode, the control unit 200 connects the switching element 100c of the branch means 100 to the connection terminal 100a. 
In the above-described linear compressor, the linear motor is operated, in a load considered in the design, at an operating frequency fc identical to a natural frequency fn of the piston computed by using a mechanical spring constant Km of a coil spring and a gas spring constant Kg of a gas spring. Accordingly, the linear motor is operated in the power mode only in the load considered in the design to improve efficiency.
Since the load is actually variable, the gas spring constant Kg of the gas spring and the natural frequency fn of the piston computed by using the same are changed.
In detail, in the design, the operating frequency fc of the linear motor is set to be equalized to the natural frequency fn of the piston in the mid load region. Even if the load is varied, the linear motor is operated at the constant operating frequency fc. However, the natural frequency fn of the piston increases with the increase of the load.
                              f          n                =                              1                          2              ⁢              π                                ⁢                                                                      K                  m                                +                                  K                  g                                            M                                                          Formula        ⁢                                  ⁢        1            
Here, fn represents the natural frequency of the piston, Km and Kg represent the mechanical spring constant and the gas spring constant, respectively, and M represents the mass of the piston.
In the design, since the ratio of the gas spring constant Kg in the total spring constant KT is small, the gas spring constant Kg is not considered or is set to have a constant value. In addition, the mass M of the piston and the mechanical spring constant Km have constant values. Therefore, the natural frequency fn of the piston is computed as a constant value by the above formula 1.
Actually, as the load increases, the pressure and temperature of the refrigerants increase in the limited space. As a result, the gas spring constant Kg increases due to the increase of an elastic force of the gas spring itself, and the natural frequency fn of the piston proportional to the gas spring constant Kg also increases.
In the conventional art, in the case that the control unit 200 controls the switching element 100c, an electric energy accumulated in the coil winding body L is operated to generate an inrush current.
If the applied power is varied, such variation changes the output of the linear compressor regardless of the control of the control unit 200. If the power is excessively applied, the linear compressor undergoes an overload or performs an abnormal operation. That is, the linear compressor is not normally operated.
The conventional control apparatus for the linear compressor as shown in FIG. 1 controls the operating frequency fc without considering the natural frequency fn of the piston or movable member varied according to the gas spring constant Kg. Even though the output of the linear compressor can be varied according to the cooling force of the load, the resonant frequency of the linear compressor is not kept. As a result, efficiency of the linear compressor decreases. Moreover, the efficiency and cooling force of the linear compressor are considerably changed due to variation of the externally-applied power. It is a fatal problem in the operation of the linear compressor.